Known methods of performing seismic surveys for formations under a body of water must overcome some unique problems, such as reflections at the air/water interface. When water depths increase beyond 30 to 40 feet, the period of the received reflection from the air/water interface and subsequent reverberations in the water column become too large for deconvolution algorithms to remove. If this energy is not removed from the processed data, reflecting horizons are unnecessarily complicated and fault planes are obscured.
Known methods employed to remove the water-column reverberations use simultaneous data from both hydrophones (pressure) and geophones (velocity) sensors. At certain places on the ocean bottom, typically called stations, a hydrophone and geophone are placed close to each other as a pair. When a water column reverberation arrives at the geophone/hydrophone pair, the hydrophone reacts with a polarity opposite that of the geophone. Therefore, when the pressure and velocity signals are summed unwanted reverberations should be significantly eliminated. However, the signals are not effectively eliminated because of the difference between signals from geophones and hydrophones. For example, at low frequencies a problem is that a geophone is a second order system with a dB roll-off approximately two times the hydrophone's dB roll-off, which is a first order system. Therefore, combining the measured response of a hydrophone with the measured response of the geophone results in poor cancellation of the unwanted reverberations.
Known methods of causing the hydrophone to behave like a second order system require using an inductance or transformer. The transformer inductance resonates with the hydrophone capacitance to produce a second order response resembling the response of the geophone in its low frequency amplitude and phase characteristics. However, using the transformer introduces additional variables that add distortion to the signal, especially at low frequencies. For example, the transformer inductance changes with signal level, thus causing a signal phase shift.
Furthermore, known methods have increased errors because the capacitance of hydrophones vary from hydrophone to hydrophone. Accordingly, the response of one hydrophone is not identical to the response of another hydrophone, even though the two hydrophones are at the same temperature.
Therefore what is needed is a method and an apparatus for modifying the response of a hydrophone to match the response of a geophone and significantly reduce response variations between hydrophones caused by inherent characteristics.